This invention relates to an improved fiber bed separator for the separation of liquid particulates (with or without solids dissolved or suspended therein) from gas streams. More particularly, it relates to an improved fiber bed separator whereby bubble re-entrainment of the collected liquid phase from the bottom of a vertically disposed fiber bed is substantially prevented thereby improving the operating characteristics of the fiber bed and allowing extension of the operating ranges of gas bed velocity and/or liquid particulates loading beyond that of prior fiber bed separators.
Fiber bed separators have found widespread use in applications wherein aerosols, particularly of less than 3 microns, must be separated from a gas or vapor (hereinafter and in the claims collectively referred to as "gas") stream. Some of the more frequent applications include removal of acid mists, such as sulfuric acid mist, in acid manufacturing processes, plasticizer mists in, for example, polyvinyl chloride floor or wall covering manufacture, water soluble solid aerosols such as, for example, emissions from ammonium nitrate prill towers. In removal of solid aerosols, the collected solid particulates are dissolved in, or flushed away by, a liquid within the fiber bed through use of an irrigated fiber bed or of a fogging spray of liquid such as water injected into the gas stream prior to the fiber bed.
Re-entrainment of collected liquid from the downstream surface of the fiber bed is often a problem with fiber bed separators and can arise from two mechanisms. As the liquid drains down through the fiber bed and/or the downstream surface thereof, the moving gas stream can cause some of the draining liquid to break or bubble out of the descending liquid stream and become re-entrained in the gas stream as droplets. This problem is particularly severe at the bottom of a vertically disposed fiber bed since all of the liquid collected by the fiber bed necessarily drains to the bottom and from a practical standpoint because of gas phase drag on the liquid, out the downstream surface at the bottom of the fiber bed. At this dis-engagement point where the greatest cumulative drainage occurs, gas phase drag causes bubbling of the draining liquid. As these bubbles break, large to sub-micron sized fragments or droplets are formed which are carried away by the moving gas stream as what is termed "bubble re-entrainment".
The second re-entrainment mechanism termed "bed re-entrainment" occurs at gas bed velocities so high that gas phase drag on the draining liquid in the entire fiber bed causes the liquid to flow in the upper reaches of the fiber bed to the downstream surface thereof with bubbling and fragmentation into re-entrainment. Thus, in a given fiber bed and at a constant liquid loading, as bed velocity increases, a point is reached where bubble re-entrainment begins at the bottom of the fiber bed and as the bed velocity is increased even further bed re-entrainment begins to occur, until with only minor increases in velocity, the bed re-entrainment is occurring from substantially the entire downstream surface of the fiber bed. This invention is directed to the elimination or substantial reduction of bubble re-entrainment. A further advantage of this invention is that with bubble re-entrainment problems solved, the fiber bed elements of this invention can be designed to operate at higher bed velocities up to where bed re-entrainment reaches unacceptable levels.